An alkaline earth metal carbonate micropowder (i.e., fine powder) is widely used as filler for polymer resins.
D1 (JP 2008-101051 A) describes a resin film containing a filler comprising an alkaline earth metal carbonate micropowder of sub-micron size or nano size is well improved in its tensile strength and handling easiness such as prevention to blocking occurring in its winding procedure, keeping its transparency. D1 further describes that the alkaline earth metal carbonate micropowder of sub-micron size or nano size can be made resistant to aggregation in an organic solvent by wet-treating surfaces of the carbonate particles dispersed in an organic solvent with a surface-treating agent having a carboxylate group and then a dispersant such as a non-ionic surface active agent.
D2 (JP 2004-35347 A) describes that a non-birefringent optic resin material can be prepared by canceling birefringence formed in a polymer resin article owing to orientation of the polymer chain therein by dispersing acicular strontium carbonate particles having a mean particle length of 500 nm or less therein under such condition that the direction of the length of the carbonate particles is aligned in parallel with or vertical to the direction of the extended polymer chain. D2 further discloses methods for producing the non-birefringent optic resin materials such as a method of spreading a polymer solution comprising a polymer resin and an acicular strontium carbonate powder in an organic solvent on a glass plate to form a polymer solution film, evaporating the solvent from the polymer solution film, and extending the polymer film, and a method of kneading an acicular strontium carbonate powder and a polymer resin to form pellets and molding the thus formed pellets in an injection machine to give a polymer resin plates.
As is described in D1 and D2, particulate or acicular alkaline earth metal micropowders comprising particles of sub-micron size or nano size have been studied for the use as fillers for polymer resins. There are problems, however, that once-dried alkaline earth metal carbonate micropowder easily forms aggregates, and hence it is difficult to disperse the dry alkaline earth metal carbonate micropowder in an organic solvent to form a dispersion in which the micropowder is dispersed in the form of primary particles or the like. In this connection, D1 describes a method for preventing formation of aggregates of the carbonate particles which are once dispersed in an organic solvent. However, D1 is silent with respect to methods for dispersing a dry alkaline earth metal carbonate micropowder in an organic solvent to give a dispersion in which the carbonate micropowder is dispersed in the form of primary particles or the like.